A semiconductor device is fabricated by semiconductor technologies such as a silicon wafer forming technology, and a logic design technology and so on. A final product through a semiconductor fabricating process is a chip of a plastic package type. The chip may be fabricated with different logics and functions according to a purpose for use thereof. Most of important semiconductor chips configuring an application system are assembled to a printed circuit board (PCB) and several driving voltages for driving the chips are supplied thereto.
All semiconductor devices including semiconductor memory operate according to input/output signals having specific purposes. Namely, operations of the semiconductor device are carried out in response to a combination signal of input/output signals. An output signal of a certain semiconductor device may be used as an input signal of another semiconductor device in the same system.
An input buffer is a part buffering a signal applied from an external circuit and providing the buffered signal into the semiconductor device and there is a static input buffer as the simplest type. The static input buffer consists of a PMOS transistor and an NMOS transistor connected in series between a power supply voltage and a ground voltage. The static input buffer has an advantage that the configuration is very simple, however, tolerance against a noise is weak, and, an input signal having large amplitude is required. Namely, the large amplitude between a logic high level and a logic low level of the input signal is required. Therefore, the static input buffer is not proper to be applied to a device, which the amplitude of input signals is small or a high operation frequency is required. Accordingly, an input buffer of a differential amplification type, which has a high tolerance against a noise and is easy to be applied to a high frequency, is suggested.
FIG. 1 is a schematic diagram illustrating a conventional input buffer of a differential amplification type. In particular, the input buffer 100 of the differential amplification type includes a differential amplification unit 11 and an internal buffer 12. The differential amplification unit 11 compares an input signal applied to a positive terminal (+) thereof with a reference voltage signal applied to a negative terminal (−) thereof. The internal buffer 12 buffers an output from the differential amplification unit 11 and issues an internal signal INT.
FIG. 2 is a circuit diagram illustrating the differential amplification unit 11 in FIG. 1. As shown, the differential amplification unit 11 includes a differential amplifier 11A and an inverter 11B. The differential amplifier 11A includes a first NMOS transistor MN1, a second NMOS transistor MN2, a first PMOS transistor MP1, and a second PMOS transistor MP2 to form a current mirror. The reference voltage Vref is applied to a gate of the first NMOS transistor MN1, and the input signal IN is applied to a gate of the second NMOS transistor MN2. The differential amplifier 11A further includes a third NMOS transistor MN3, which is coupled between ground and the first and second transistors MN1 and MN2. A buffer enable signal is applied to the gate of the transistor MN3 for a bias. Third and fourth PMOS transistors MP3 and MP4 forming a current mirror are coupled to the first and second transistors MP1 and MP2, respectively, in parallel. The buffer enable signal buffer_en is applied to the gates of the third and fourth PMOS transistors MP3 and MP4.
The inverter 11B includes a pull-up PMOS transistor MP5 and a pull-down NMOS transistor MN4. An output signal of the differential amplifier 11A is commonly applied to the gates of the pull-up PMOS transistor MP5 and a pull-down transistor MN4.
FIG. 3 is a circuit diagram illustrating the internal buffer 12 in FIG. 1. The internal buffer 12 includes a first inverter INV1 and a second inverter INV2 coupled in series. The number of the inverters in the internal buffer 12 may be adjusted.
For example, an operation of the input buffer of the differential amplification type will be described by referring to FIGS. 2 and 3. When the input signal IN is applied to an input terminal of the differential amplifier 11A, the differential amplifier 11A will be operated according to states whether the voltage level of the input signal IN is higher or lower than that of the reference voltage signal Vref because since the voltage of the input signal IN is higher or lower than the reference voltage signal Vref. The reference voltage signal Vref is a static voltage, which always maintains a constant voltage level. The reference voltage signal Vref may be provided through a specific input pin from an external circuit or may be generated in the semiconductor device.
A constant current I1 flows at the first NMOS transistor MN1 receiving the reference voltage signal Vref. A current flowing at the second NMOS transistor MN2, which is symmetrically configured with the first NMOS transistor MN1, is determined by the voltage level of the input signal IN. In particular, the differential amplifier 11A determines a voltage level of an output node N1 according to a comparison result between the currents I1 and I2.
If the current I2 is relatively higher than the current I1, the voltage level of the output node N1 decreases. Finally, a signal of a logic high level is outputted via the inverter 11B. In this case, the input buffer 100 issues a signal of a logic high level. On the other hand, if the current I2 is relatively lower than the current I1, an output of the inverter 11B becomes a signal having a logic low level. In this case, the input buffer issues a signal of a logic low level. The buffer enable signal buffer_en is disabled to a logic low level at a power-down mode or a self-refresh mode so that the input buffer is inactivated to a precharge state and, in other modes, the buffer enable signal buffer_en is adjusted to a logic high level so that the input buffer is activated.
The input buffer 100 basically plays a role of an interface for transmitting signals between an internal circuit and an external circuit of a semiconductor device. Therefore, an input buffer configured to rapidly transmit signal variation at an input terminal is required. The semiconductor device performing an amplification operation with high gain is suggested to rapidly transmit signals.
Because the reference voltage signal Vref having a constant potential level is always supplied to the gate of the first NMOS transistor MN1 in the input buffer 100 of the differential amplification type illustrated in FIGS. 1 to 3, the current I1 flowing at the first NMOS transistor MN1 is fixed. Therefore, high amplitude of the input signal IN is required for a rapid operation. Generally, however, small amplitude of an input signal is required to improve signal integration and rapidly vary a voltage level of a signal in a system operated with a high speed.